Green’s Dictionary of Slang

shooter n.3

[shot n.1 (6a)]

1. a martini and pernod cocktail.

[US]K. Brasselle Cannibals 157: I’ll make us both another shooter.

2. (US) a measure of spirits, esp. whisky; tequila.

[US]S. Morgan Homeboy 97: Geez, he could use a shooter of whiskey.
[US]J. Wambaugh Finnegan’s Week 210: The ox just wanted to drink tequila shooters [...] He signaled for another shooter.